Polyimide (PI) fiber as one type of high-performance fiber has high strength, high modulus, resistance to high temperature, low temperature and radiation and other high performances in addition to good biocompatibility and dielectric properties. Extensive application is expected in the fields of atomic energy industry, space environment, wrecking, aerospace, national defense, new-type buildings, high-speed vehicles, oceanic development, sports equipment, new energy, environmental industry and protection equipment.
The current methods for preparing PI fiber mainly include one-step method and two-step method. The technical route of the one-step method: A PI solution is used as a spinning solution. PI fiber is spin by wet method or dry-wet method. After preliminary drawing, the fiber possesses certain strength. After the solvent is removed, thermal drawing and thermal treatment (300° C.-500° C.) is conducted. High-strength high-modulus PI fiber may be obtained. This method features a simple spinning process, but in view of the current synthesis of PI, the common solvents are phenols. Phenol solvents (such as: cresol and p-chlorophenol) not only have high toxicity but also have a large residual amount in fiber. They can hardly be thoroughly removed. It is not good for environmental protection, resulting in difficulty in industrialization. Moreover, the technology of one-step method has very high requirement on the solubility of PI. This greatly reduces the corrosion resistance and heat resistance of PI fiber. Chinese invention patent ZL O2112048.X and American patents U.S. Pat. No. 4,370,290 and U.S. Pat. No. 5,378,420 all disclose a method for preparing PI fiber by one-step method. The technical route of two-step method: Firstly, the concentrated solution of polyamic acid (PAA) is sprayed by wet method or dry-wet method to obtain PAA fiber. Then the PAA fiber obtained in the first step is chemically or thermally cyclized to obtain PI fiber. For example, Japanese published unexamined patent applications JP3287815 and JP4018115 both adopt this method to prepare PI fiber. The advantage of this method: It solves the processing difficulty caused by the infusibility and insolubility of PI fiber, the synthetic raw materials and solvents have many types and low toxicity. The residual amount of the solvents in the fiber is low. It is suitable for industrial production. The disadvantage of this method: The mechanical property of PI fiber prepared by this method is low in general.
Chinese patent (Application No.: 200710050651.1) discloses a PI fiber with a benzimidazole structure and its preparation method. In this method, PAA spinning solution is prepared from 2-(4-aminophenyl)-1H-benzimidazol-5-amine (BIA) and diandhydrides at a molar ratio of 1:1, then the PAA spinning solution is spun to obtain PAA precursor and in the end the PAA precursor is thermally imidized to obtain PI fiber. Its tensile strength is 0.73˜1.53 GPa and initial modulus is 45.2˜220 GPa. Chinese patent application (application No.: 201010572496.1) discloses a PI fiber made from 3,3′,4,4′-biphenyl tetracarboxylic diandhydride (BPDA), p-phenylenediamine (pPDA) and 2-(4-aminophenyl)-1H-benzimidazol-5-amine (BIA) and its preparation method, specifically: p-PDA and BIA with a molar ratio of 0.8˜0.95:0.05˜0.2 and BPDA are dissolved in a solvent to obtain a PAA spinning solution. Then the PAA spinning solution is spun to obtain PAA fiber. Then the PAA fiber is dried, thermally cyclized and thermally drawn to obtain PI fiber. Its strength is 2.5 GPa. In the foregoing two methods, the mechanical property of PI fiber is improved both by adding BIA. Although the mechanical property is improved remarkably, it sill does not meet the performance requirements of high-strength high-modulus polyimide. Greater breakthrough and change in synthesis method and preparation process are needed.